Environmental monitoring requires the measurements of pollutants at trace concentrations (ppm to ppt), because even at these levels they pose a threat to human health and to the environment. A variety of conventional laboratory based analytical techniques are used for pollution monitoring. Currently, gas chromatographs, mass spectrometers and Fourier transform infrared spectrometers (FTIR) are the most commonly used instruments. These techniques have excellent merits in terms of sensitivity, detection limit and other performance characteristics. However, they are relatively large, expensive and do not lend themselves to easy portability.
The increasing needs for inexpensive, small monitoring devices have added new impetus to miniaturize analysis systems. It is well known that miniaturization offers functional and economic benefits such as the reduction in sample size, decrease in reagent consumption and inexpensive mass production. Advancements in thin-film technologies have expanded the range of possible microsensor designs. On the other hand, micromachining processes, particularly anisotropic and plasma etching, and the sacrificial layer method make possible the construction of a variety of three-dimensional structures. It is feasible to employ these methods to produce sophisticated, low power integrated sensing systems at a modest cost. The high degree of reproducibility and the relatively small size of these devices enhance both performance and the potential for practical applications.
A few types of microsensors have been developed to date. Tin-oxide-based sensors have been widely used in gas sensing. An important environmental application is the detection of low concentration toxic gases (i.e., CO, NO2, O3 etc.). SnO2 films are commonly used as gas sensors due to their high sensitivity to different gases, low production cost, and the ease of use. Surface acoustic wave (SAW) sensors are another widely used class of highly sensitive environmental sensors. A coated SAW device acts as a chemical sensor by adsorbing analytes on its surface. A mass loading on the surface results in a change in propagation velocity and a corresponding phase shift. Schottky-diode-type sensors have also been used in gas sensing, in which, when an analyte diffuses towards the interface between the metal and the insulting layers of a diode, the height of the Schottky barrier diminishes, leading to a change in either the forward voltage or the reverse current.
Chemical species can be detected using electrochemical sensors. An example of a solid electrolyte electrochemical sensor is the ZrO2—based high-temperature oxygen sensor. This sensor is operated at 650° C. to ensure the ionic conductivity of ZrO2.
Micromachined gas chromatographs have also been developed. GC columns have been etched on silicon, and diaphragm based valves have been developed as GC injectors. Micromachined thermal conductivity detectors have been successfully made, and are commercially available.
In principle, sensors and other micro devices can provide real-time (or near real-time), on-line measurements. It is desirable that they be completely automated, and not require additional chemical reagents or sample preconditioning. However, the absence of memory effects, high sensitivity, selectivity, reproducibility, short response time and long-term stability are prerequisite for their real-world applicability. The limited success of microsensors are due to the inability to meet some of these requirements. In trace analysis, such as in environmental monitoring, the biggest drawback has been the low sensitivity, and the high detection limits of the sensors.
One way to enhance sensitivity in any measurement is to provide some kind of preconcentration. The key component in trace analysis is the concentration step where the analytes are accumulated before the analysis. Sorbent trapping in air sampling, solid phase extraction and SPME are common examples of preconcentration. This allows a larger amount of analyte to be concentrated and then released into a detection device. Larger sample throughput in terms of the mass of analyte per unit time results in a higher signal to noise ratio.
A small sorbent trap known as a microtrap has been employed as a concentration plus injection device for continuous monitoring of organics in gas streams by gas chromatography, mass spectrometry, or by a non-methane organic carbon (NMOC) analyzer. Sample passes continuously through the microtrap, and periodic electrical heating releases the adsorbed analytes as a “concentration pulse”, which serves as an injection for the detection system. Its small size allows it to be cycled at high frequency, and the preconcentration effect allows ppb level detection.
However, as yet there has been no way to provide a preconcentration microdevice that may be employed in conjunction with a microsensor.